A conventional voltage source inverter includes an AC/DC converter and a DC/AC inverter interconnected by a DC-link. This voltage-source is controlled on the inverter side to generate an AC current output of selected frequency and under selected voltage output, thereby to control an AC load in response to a predetermined constant voltage constant frequency AC power supply at the input of the AC/DC converter, while generally a six-thyristor bridge is used for AC/DC conversion and also six power switches are used mounted in a bridge to form the inverter. Control of the inverter is done in different ways, for instance using pulse-width-modulation to control the inverter switches in succession, thereby to generate AC output waves of controlled magnitude and frequency which can be used for the respective phases of an AC motor, for instance.
Another approach in the prior art to generate a multiphase variable frequency variable voltage output from a constant frequency constant-voltage AC input, is the frequency changer, more specifically the unrestricted frequency changer (UFC), as described in U.S. Pat. Nos. 3,470,447 and 3,493,838 of L. Gyugyi et al. In such case, bilateral switches are used on each phase of the AC input to connect with each phase at the AC output, while providing current flow in both directions.
The bilateral switches are preferably devices which are capable of being triggered ON at will and of being interrupted at will, by triggering to permit current flow in both directions.
The bilateral switches select segments of the supply voltage for application to the load, and in a three-phase power supply, the selection process involves sequentially reconnecting the load to the supply in such a way that a defined phase shift (60 degrees for six-pulse, 120 degrees for three-pulse) is produced by each reconnection. For each rate of progression through this sequence a different fundamental output frequency is obtained. This system can be used to produce a balanced set of controlled-frequency output voltages, of constant amplitude, which have relatively low harmonic distortion. In order to obtain conrol of the fundamental output voltage, schemes have been suggested, most of which involve reducing the load voltage to zero periodically, with controlled duty cycle. In general, six-pulse UFC's operated in this manner have been shown to be technically viable. However, using only half as many switches in a three-pulse version is tempting, but has been less acceptable.
The gist of the present invention rests upon the realization that the UFC, as just described, is essentially a controlled source of AC voltage, much like a conventional voltage-source inverter. Here too, the amplitude and frequency are controlled variables, despite the fact that modern control schemes for AC motor drives, which require power amplifiers and control of the instantaneous output voltage to effect closed-loop current control, are generally not applicable to UFC systems.